[Globals] Treat nobuiltin fns as maybe-derefined.

Callsites could be marked as `builtin` while calling `nobuiltin`
functions. This can lead to problems, if local optimizations apply
transformations based on the semantics of the builtin, but then IPO
treats the function as `nobuiltin` and applies a transform that breaks
builtin semantics (assumed earlier).

To avoid this, mark such functions as maybey-derefined, to avoid IPO
transforms on them that may break assumptions of earlier calls.

Fixes #57075
Fixes #48366

Reviewed By: ychen

Differential Revision: https://reviews.llvm.org/D97735